Method for manufacturing silicon wafer

ABSTRACT

Provided is a process for manufacturing a silicon wafer employing heat treatment which is applied on the silicon wafer in inert gas atmosphere represented by Ar annealing to annihilate Grown-in defects in a surface layer region of the silicon wafer as well as to cause no degradation of haze and micro-roughness on a surface thereof. In a process for manufacturing a silicon wafer having a step of heat treating the silicon wafer in inert gas atmosphere, using a purge box with which the silicon wafer heat treated in the inert gas atmosphere can be unloaded to outside a reaction tube of a heat treatment furnace without being put into contact with the open air, the purge box is filled with mixed gas of nitrogen and oxygen or 100% oxygen gas, and the heat treated silicon wafer is unloaded into the purge box.

TECHNICAL FIELD

The present invention relates to a process for manufacturing a siliconwafer (hereinafter also simply referred to as a wafer) employing heattreatment for annihilating Grown-in defects in a surface layer region ofthe silicon wafer and causing no degradation of haze and micro-roughnesson a surface thereof.

BACKGROUND ART

While it has been known that in a CZ silicon wafer there are crystaldefects referred to as so called Grown-in detects such as COP (CrystalOriginated Particle) and an oxide precipitate, heat treatment performedin hydrogen atmosphere (hereinafter also referred to as hydrogenannealing) has been proposed as a method for annihilating Grown-indefects in the vicinity of a wafer surface. This heat treatment requiresuse of hydrogen at a high temperature of 1000° C. or higher; therefore,a safety measure is required, and an ordinary open furnace (for example,a furnace unsealed on the furnace opening side such as a horizontalfurnace) cannot be used for the treatment, so that it is necessary toprovide a sealing structure for enhancing air-tightness and anexplosion-proof facility as a countermeasure against explosion,resulting in very high cost.

On the other hand, it has been recently found that the Grow-in defectscan be annihilated in heat treatment carried out in argon atmosphere(hereinafter also referred to as Ar annealing) as in the hydrogenannealing. Since the Ar annealing has no explosiveness, a saferoperation is ensured as compared with the hydrogen annealing, but it hasalso been known that the Ar annealing displays a characteristic behaviorto a silicon wafer in contrast to the safety operation. There is givenas an example of the characteristic behavior a fact that tiny pits areeasily formed on a surface of a wafer subjected to the Ar annealing.

This pit formation is described as follows: An oxide film is formed byoxygen and water as very small amounts of impurities included in rawmaterial gas or oxygen and water in the open air involved through theopening of a reaction tube during a heat treatment step or whenunloading a wafer, and the oxide film further reacts with silicon (Si)according to a reaction of SiO₂+Si→2SiO; consequently Si is etched, theetched sites being observed as pits. The pits contribute to degradationof local surface roughness (micro-roughness) and long periodic surfaceroughness (haze) on a wafer surface. Thus, Ar gas is sensitive to atrace of impurities, and small environmental changes such as temperaturefluctuations, so it has the demerit of difficulty in handling.

There are proposed the following methods for preventing such degradationof surface roughness of a wafer in the Ar annealing: one is to reduce awater concentration in raw material gas and the other is to form anetching-resistant film by treating the wafer in oxygen or nitrogenatmosphere prior to unloading it from a heat treatment furnace after theAr annealing (JP A 93-299413).

As described in the above published patent application, however, while anitride film is produced in nitrogen treatment at 1000° C. or higher,the nitride film produced at 1000° C. or higher has a very slow speed inHF etching, compared with an ordinary natural oxide film, and is noteasily etched in cleaning with SC1 (a mixture of NH₄OH/H₂O₂/H₂O), SC2 (amixture of HCl/H₂O₂/H₂O) or the like; therefore, it affects heattreatment in the next step. In addition, since a nitride film has adielectric constant higher than an oxide film, in other word, a higherelectrostatic capacitance, charged particles are easy to attach theretoand particles that have been once attached thereto are hard to beremoved. This is a drawback when a nitride film is grown on a wafersurface.

On the other hand, tiny pits are also easily generated when switching Argas to oxygen gas prior to unloading a wafer after the Ar annealing.This is because, if oxygen of a prescribed amount or higher is notpresent in Ar gas (in other words, an oxygen concentration of aprescribed amount or lower), a region where an oxide film is formed isetched by the following reaction:SiO₂+Si→2SiO.This phenomenon inevitably occurs as a transitional one when switchingAr to oxygen. Such etching occurs when a partial pressure of oxygen is aprescribed value or lower even if oxygen is introduced after Ar isremoved by a vacuum pump.

DISCLOSURE OF THE INVENTION

There occurs the problem that pits are easily generated by etching whenswitching Ar gas to oxygen gas not only in Ar annealing but also inannealing using other inert gas atmosphere or inert gas atmosphereincluding hydrogen at a content of the explosion limit (4%) or less(hereinafter simply referred to inert gas atmosphere as a general term).A wafer surface is degraded in terms of haze and micro-roughness theinstant that the pits are generated. Moreover, it has been known thatthe micro-roughness affects an oxide film dielectric breakdown strengthand mobility of electrons and holes directly under an oxide film of atransistor of a MOS structure.

Especially, as a drive frequency of a MOS transistor is higher, it isnecessary to increase a mobility of carriers (electrons and holes).Furthermore, if a pit is present, concentration of an electric fieldoccurs at the pit; there take place increase in leakage current anddegradation in oxide film dielectric breakdown strength. Under thesecircumstances, it is necessary to reduce the pits of the Ar annealedwafer and improve haze and micro-roughness thereof.

The present invention has been made in view of the problem inherent tothe prior art mentioned above, and its object is to provide a processfor manufacturing a silicon wafer employing heat treatment forannihilating Grown-in defects in a surface layer region of the siliconwafer causing no degradation of haze and micro-roughness on a surfacethereof, the heat treatment being applied on the silicon wafer in inertgas atmosphere represented by Ar annealing.

To solve the problem mentioned above, the present invention provides, ina first aspect, a process for manufacturing a silicon wafer having astep of heat treating the silicon wafer in inert gas atmosphere, whereinusing a purge box with which the silicon wafer heat treated in the inertgas atmosphere can be unloaded to outside a reaction tube of a heattreatment furnace without being put into contact with the open air, thepurge box is filled with mixed gas of nitrogen and oxygen or 100% oxygengas, and the heat treated silicon wafer is loaded into the purge box.

In a second aspect, the present invention provides a process formanufacturing a silicon wafer having a step of heat treating the siliconwafer in inert gas atmosphere, wherein when the silicon wafer heattreated in the inert gas atmosphere is unloaded to outside a reactiontube of a heat treatment furnace, the silicon wafer is unloaded whilebeing blown by mixed gas of nitrogen and oxygen or 100% oxygen gas.

In a third aspect, the present invention provides a process formanufacturing a silicon wafer having a step of heat treating the siliconwafer in inert gas atmosphere, wherein the heat treatment is performedin a condition that a relationship between an exhaust pressure P (mmH₂O)and a gas flow rate F (SLM) in a heat treatment furnace during the heattreatment in the inert gas atmosphere satisfies the following equation(1):F≧(−25/P)+2.5  (1).

The shortest distance between a reaction tube of the heat treatmentfurnace and the silicon wafer is preferably in the range of from 10 mmto 50 mm. The reason why the upper limit of the shortest distancebetween the reaction tube and the silicon wafer is 50 mm is that ifexceeding 50 mm, a distance between the wafer and the reaction tube (aquartz tube) is excessively large, so the quartz tube having aconsiderably large inner diameter relative to the wafer to be heattreated is required; there arise too big demerits such as scaling up ofan apparatus and low cost-efficiency to be practical.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a descriptive view showing one embodiment of a first aspect ofa process for manufacturing a silicon wafer of the present invention;

FIG. 2 is a descriptive view showing one embodiment of a second aspectof a process for manufacturing a silicon wafer of the present invention;

FIG. 3 is a descriptive sectional view showing a positional relationshipbetween a wafer and a quartz tube in one embodiment of a third aspect ofa process for manufacturing a silicon wafer of the present invention,wherein part (a) is of a case where the centers of the wafer and thequartz tube are in agreement with each other and part (b) is of a casewhere the centers are not in agreement with each other;

FIG. 4 is a graph showing a relationship between an oxygen concentrationin mixed gas and haze on a surface of a wafer heat treated inExperimental Example 1;

FIG. 5 is a graph showing a relationship between an oxygen concentrationin mixed gas to be blown against an unloaded wafer and haze on a surfaceof the heat treated wafer in Experimental Example 2;

FIG. 6 is a graph showing a relationship between an exhaust pressure Pand a gas flow rate F during heat treatment in Experimental Example 3;and

FIG. 7 is a graph showing a relationship between the shortest distancefrom a wafer to a quartz tube and haze on a surface of a wafer heattreated in Experimental Example 4.

BEST MODE FOR CARRYING OUT THE INVENTION

Description will be given of embodiments of the present invention belowtogether with the accompanying drawings and it is needless to say thatvarious modifications or alterations in addition to the examples shownin the figures can be performed as far as not departing from thetechnical concept of the present invention.

FIG. 1 is a descriptive view showing one embodiment of a first aspect ofa process for manufacturing a silicon wafer of the present invention. InFIG. 1, a heat treatment furnace 1 is of a vertical type. The heattreatment furnace 12 has a reaction tube (a quartz tube) 12 a, in whichmany wafers W are disposed in a wafer boat 13. On the side of a furnaceopening section 14 a purge box 16 is provided. The purge box 16 is shutoff from heat treatment atmosphere 18 during heat treatment of thewafers W. When a shutter 20 at a connecting section is opened in case ofunloading the heat treated wafers W, heat treatment atmosphere 18 andatmosphere 22 in the purge box 16 are communicated with each other. Thepurge box 16 is filled with mixed gas of nitrogen and oxygen before acover 20 is opened and the heat treated wafers W can be loaded into thepurged box 16 filled with the mixed gas atmosphere 22. In FIG. 1, areference numeral 15 designates a process (raw material) gas supply pipeand a reference numeral 17 designates an exhaust pipe.

FIG. 2 is a descriptive view showing one embodiment of a second aspectof a process for manufacturing a silicon wafer of the present invention.A heat treatment furnace 12 in FIG. 2 is also of a vertical type. Sincea structure of the heat treatment furnace 12 is similar to FIG. 1, thesecond description thereof is omitted. A gas supply pipe 24 is providedto a furnace opening section 14 of the heat treatment furnace 12 apartfrom the supply pipe 15 for the process gas (Ar gas). As shown in FIG.2, while moving the wafer boat 13 downward in order to unload the wafersW, mixed gas 26 is supplied in approximately parallel with a wafersurface from the gas supply pipe 24. With the above construction,invasion of atmospheric air into the heat treatment furnace 12 can beeffectively prevented as well as a protective film can be formed on asurface of each of the wafers W.

FIG. 3 is a descriptive sectional view showing a positional relationshipbetween a wafer and a quartz tube in one embodiment of a third aspect ofa process for manufacturing a silicon wafer of the present invention,wherein part (a) is of a case where the centers of the wafer and thequartz tube are in agreement with each other and part (b) is of a casewhere the centers are not in agreement with each other.

As described above, a process for manufacturing a silicon wafer of thethird aspect of the present invention is to perform heat treatment in acondition that a relationship between an exhaust pressure P (mmH₂O) anda gas flow rate F (SLM) in the heat treatment furnace 12 satisfies thefollowing equation (1):F≧(−25/P)+2.5  (1).

Here, as shown in FIGS. 3(a) and (b), it was found that a distancebetween the outer peripheral end of each of the wafers W supported bythe wafer boat 13 and the inner wall of the quartz tube 12 a affects ahaze level of each of the heat treated wafers W, which is shown as aconcrete example in Experimental Example 4 described later.

As shown in FIG. 3(a), if a wafer W is set in the almost central portionof the quartz tube 12 a, a distance D between the outer peripheral endof the wafer W and the inner wall of the quartz tube 12 a is almostequal at any position. On the other hand, if a position of a wafer W setin the quartz tube 12 a is displaced (the centers of the wafer W and thequartz tube 12 a are displaced from each other), the shortest distance dbetween the wafer W and the quartz tube can be obtained. By setting theshortest distance d to a value in the range of from 10 mm to 50 mm,degradation of a haze level on the heat treated wafer W can beprevented.

Moreover, if a method for unloading a wafer in the first or secondaspect is applied when the heat treated wafer is unloaded in the thirdaspect of the present invention, it is more effective for preventingdegradation of a haze level on a wafer.

Subsequently, description will be given of the present invention in amore concrete manner taking up experimental examples.

EXPERIMENTAL EXAMPLE 1

Using silicon mirror wafers having a diameter of 150 mm, a conductivitytype of p-type, a crystal axis orientation of <100> and a resistivity of100 Ω·cm, heat treatment was performed in 100% argon atmosphere at 1200°C. for 60 min. A vertical furnace provided with a purge box similar toFIG. 1 was used.

In this Experimental Example, an influence on haze was investigatedchanging the mixing ratio of mixed gas composed of nitrogen and oxygen.In the heat treatment, a flow rate of argon gas was 20 SLM, an exhaustpressure was −5 mmH₂O and an unloading temperature was 800° C.

Haze measurement was performed with Surfscan SP1 made by KLA-tencorCorp. This measuring instrument is operated such that a wafer surface isscanned with laser light, a scattered light intensity is measured andthe scattered light intensity is obtained in the unit of ppm relative toincident light. The results of the haze measurement are shown in FIG. 4.

From the results of FIG. 4, it has been found that in mixed gasatmosphere of nitrogen and oxygen or 100% oxygen atmosphere, a hazelevel is low, and the lower the oxygen concentration is, the lower thehaze level becomes. However, a haze level of a wafer treated with 100%nitrogen atmosphere when unloading it was extremely high as comparedwith other conditions.

EXPERIMENTAL EXAMPLE 2

Instead of the method with the purge box in Experimental Example 1,using a method wherein as shown in FIG. 2 when unloading a wafer a mixedgas stream of oxygen and nitrogen was blown against the got out wafer,such an experiment as Experimental Example 1 was conducted toinvestigate haze levels of the got out wafers.

In this Experimental Example, an influence on haze was investigatedchanging the mixing ratio of mixed gas blown against the unloaded wafer.The results of the haze measurement are shown in FIG. 5.

From the results of FIG. 5, it has been found that if the blowing gasstream is of mixed gas of nitrogen and oxygen or 100% oxygen atmosphere,a haze level is low, and the lower the oxygen concentration is, thelower the haze level becomes. However, a haze level of a wafer treatedwith 100% nitrogen atmosphere when unloading it was extremely high ascompared with other conditions.

As described above, both of Experimental Examples 1 and 2 showed thephenomenon that in mixed gas of nitrogen and oxygen or 100% oxygenatmosphere, a haze level is low, and in 100% nitrogen atmosphere, a hazelevel is high. The following is considered as the cause of the abovephenomenon.

The reason why tiny pits are easy to occur when switching the gas fromAr gas to oxygen gas before unloading the Ar annealed wafer is thatsince oxygen of a prescribed amount or more is not present in Ar gas asdescribed above, in a region where an oxide film is formed the followingreaction occurs to etch the region:SiO₂+Si→2SiO.This phenomenon inevitably occurs as a transitional one when switchingthe gas from Ar to oxygen. Therefore, it is estimated that as in theExperimental Examples 1 and 2, using a method wherein a wafer is loadeddirectly into the atmosphere containing oxygen of a certainconcentration (a partial pressure) with the purge box or another methodwherein a gas stream containing oxygen is directly blown against awafer, a sufficient protective oxide film is formed by oxygen gas at acertain partial pressure; therefore, a haze level is effectivelyreduced, while using 100% nitrogen atmosphere instead of the aboveatmosphere, a nitride film as a protective film is insufficiently formedbecause of an unloading temperature of 800° C. and water as a trace ofimpurity included in the nitrogen gas causes the above reaction locally;a haze level is degraded in this Experimental Example. Furthermore, whenan oxygen concentration of mixed gas atmosphere of nitrogen and oxygenis less than 1%, it has been experimentally confirmed that whiledegradation in haze level does not occur sometimes, there increasevariations in haze level on a wafer and between heat treated batches.Therefore, an oxygen concentration in mixed gas atmosphere is preferably1% or higher.

EXPERIMENTAL EXAMPLE 3

Using silicon mirror wafers having a diameter of 150 mm, a conductivitytype of p-type, a crystal axis orientation of <100> and a resistivity of10 Ω·cm, heat treatment was performed in 100% argon atmosphere at 1200°C. for 60 min, a gas flow rate and an exhaust pressure in the heattreatment having been changed. There was used a heat treatment furnaceof a general vertical type and the loading and unloading temperature ofa wafer was 600° C. An inner diameter of a quartz tube of the used heattreatment furnace is 220 mm and a wafer is set so as to be in thecentral portion thereof.

An exhaust pressure during the heat treatment is defined as a pressuredifferential (a pressure differential relative to the atmosphericpressure) between a pressure sensor installed in the vicinity of anexhaust port of the furnace and a pressure sensor installed outside thefurnace. As the experimental conditions, a gas flow rate F was set inthe range of from 3 to 30 (SLM) and an exhaust pressure P was set in therange of from −5 to −25 (mmH₂O). Note that 1 SLM (Standard Liter perMinute) designates a flow rate when a gas of 1 liter in the standardcondition flows for 1 min.

The heat treated wafer was observed under the collimated light and ahaze level was investigated according to whether or not cloudiness wasobserved in a peripheral portion of the wafer. The results of theinvestigation are shown in Table 1. In addition, the results in Table 1were plotted to make a graph to obtain a boundary line between thepresence or absence of cloudiness in the peripheral portion, with thefinding that the boundary line is expressed approximately by thefollowing equation (FIG. 6):F=(−25/P)+2.5.

Note that while the reason why a difference in occurrence of thecloudiness in the wafer peripheral portion exists according to arelationship between a gas flow rate and an exhaust pressure is notclear, it is considered that a gas flow in the vicinity of the waferperipheral portion affects pit formation due to etching. That is, it isestimated that if a negative pressure of an exhaust pressure relative tothe atmospheric pressure becomes higher, a gas flow smoothes even at asmaller gas flow rate, while if the negative pressure is low, gas is aptto be stagnant in a vortex in the vicinity of the peripheral portionsbetween the wafers unless a gas flow rate is increased to a certainextent; therefore, pits are easily formed only in the wafer peripheralportion.

TABLE 1 Exhaust pressure P (mmH₂O) −25 −20 −15 −10 −5 Gas flow 3 X X X XX rate F 5 ◯ ◯ ◯ ◯ X (SLM) 10 ◯ ◯ ◯ ◯ ◯ 20 — — ◯ ◯ ◯ 30 — — — — ◯

Evaluation by observation in Table 1 is defined as follows: O; nocloudiness in the peripheral portion, X; occurrence of cloudiness in theperipheral portion, and -; no experiment performed.

EXPERIMENTAL EXAMPLE 4

Two parameter values F=10 SLM and P=−15 mmH₂O were selected amongcombinations of the exhaust pressures P and the gas flow rates F inTable 1 of Experimental Example 3 and a relationship between theshortest distance from a wafer to a quartz tube and a haze level wasinvestigated. Specifically, since in Experimental Example 3, a wafer wasset almost in the central portion of a quarts tube, a distance D betweenan outer peripheral end of the wafer and an inner wall of the quartztube was almost uniformly 35 mm at any point as shown in FIG. 3(a),while in this Experimental Example, a position at which a wafer was setin the quartz tube was displaced (the centers of the wafer and thequartz tube was displaced from each other) to thereby change theshortest distance d between the wafer and the quartz tube in the rangeof from about 2.5 to 30 mm. Under the various shortest distances thewafers were heat treated. Haze levels on the heat treated wafers wereinvestigated. The results of the investigation are shown in FIG. 7.

From the results of FIG. 7, it has been found that a haze level on theheat treated wafer is related to the shortest distance between the waferand the quartz tube and the haze level on the wafer is extremelydegraded at the distance of 10 mm or less. While a cause of thisphenomenon is not clear, it is expected that a gas flow in the vicinityof a wafer peripheral portion exerts an effect on pit formation due toetching as in Experimental Example 3.

CAPABILITY OF EXPLOITATION IN INDUSTRY

As described above, according to the present invention, an effect can beachieved that heat treatment in inert gas atmosphere represented by Arannealing is applied on a silicon wafer to annihilate Grown-in defectsin a wafer surface layer region as well as to cause no degradation ofhaze and micro-roughness on a surface thereof.

1. A process for manufacturing a silicon wafer having a step of heattreating the silicon wafer in inert gas atmosphere, wherein the heattreatment is performed in a condition that a relationship between anexhaust pressure P (mmH₂O) and a gas flow rate F (SLM) in a heattreatment furnace during the heat treatment in the inert gas atmospheresatisfies the following equation:F≧(−25/P)+2.5 wherein said exhaust pressure P is defined as a pressuredifferential between a pressure sensor installed at an exhaust port ofthe heat treatment furnace and a pressure sensor installed outside theheat treatment furnace.
 2. A process for manufacturing a silicon waferaccording to claim 1, wherein in the heat treatment, the shortestdistance between a reaction tube of the heat treatment furnace and thesilicon wafer is in the range of from 10 mm to 50 mm.
 3. A process formanufacturing a silicon wafer according to claim 1, including using apurge box with which the silicon wafer heat treated in the inert gasatmosphere is unloaded to outside a reaction tube of the heat treatmentfurnace without being put into contact with the open air, the purge boxis filled with mixed gas of nitrogen and oxygen or 100% oxygen gas, andthe heat treated silicon wafer is loaded into the purge box.
 4. Aprocess for manufacturing a silicon wafer according to claim 1,including when the silicon wafer heat treated in the inert gasatmosphere is unloaded to outside a reaction tube of the heat treatmentfurnace, the silicon wafer is unloaded while being blown by mixed gas ofnitrogen and oxygen or 100% oxygen gas.
 5. A process for manufacturing asilicon wafer according to claim 1, wherein the heat treatment of thesilicon wafer in inert gas atmosphere is performed at a high temperatureof 1000° C. or higher.
 6. A process for manufacturing a silicon waferaccording to claim 1, wherein the heat treatment of the silicon wafer ininert gas atmosphere is performed to a plurality of the silicon wafersas a batch.